Trying to create a custom picker in Xamarin Forms

I'm trying to create a custom picker with 3 components. I followed the tutorial on the Xamarin site here. I got the control to work, except when I click the Done button on the picker (This is the default done button). I get the following exception:

System.InvalidCastException: Unable to cast object of type 'myproj.iOS.MyPickerModel' to type 'Xamarin.Forms.Platform.iOS.PickerRenderer+PickerSource'.

Here is my code:

In the shared proj:

public class MyPicker: Picker {}
In the iOS proj:

[assembly: ExportRenderer(typeof(MyPicker), typeof(MyPickerRenderer))]

public class MyPickerRenderer: PickerRenderer
{
// Override the OnElementChanged method so we can tweak this renderer post-initial setup
protected override void OnElementChanged(ElementChangedEventArgs e)
{
base.OnElementChanged(e);

    if (Control != null)
    {   
        var picker = (UIPickerView)this.Control.InputView;
        picker.BackgroundColor = UIColor.White;
        picker.Model = new MyPickerModel();
    }

And the Model:

public class MyPickerModel : UIPickerViewModel
{
private string[] array1 = new string [] {
"1","2","3","4"
};
private string[] array2 = new string [] {
"1a","2a","3a","4a"
};

private string[] array3  = new string [] {
    "a","b","c","d"
};

public override nint GetComponentCount (UIPickerView pickerView)
{
    return 3;
}

public override nint GetRowsInComponent (UIPickerView pickerView, nint component)
{
    // Returns
    switch (component) {
        case 0: return array1.Length;
        case 1: return array2.Length;
        case 2: return array3.Length;
        default:break;
    }
    return 0;
}

public override string GetTitle (UIPickerView pickerView, nint row, nint component)
{
    // Returns
    switch (component)
    {
        case 0: return array1[row];
        case 1: return array2[row];
        case 2: return array3[row];
        default: break;
    }
    return null;
}

public override nfloat GetComponentWidth (UIPickerView pickerView, nint component)
{
    switch (component)
    {
        case 0: return 100.0f;
        case 1: return 100.0f;
        case 2: return 100.0f;
        default: break;
    }

    return 0;
}

public override nfloat GetRowHeight (UIPickerView pickerView, nint component)
{
    return 40f;
}

}
And lastly my page:

public class MyPage : ContentPage
{
public MyPage()
{
MyPicker picker = new MyPicker
{
Title = "Color",
VerticalOptions = LayoutOptions.CenterAndExpand
};

    picker.SelectedIndexChanged += (sender, args) =>
    {

    };

    var mainLayout = new StackLayout
    {
        Orientation = StackOrientation.Vertical,
        Children =
        {
            picker

        }
    };


    // Accomodate iPhone status bar.
    this.Padding = new Thickness(0, Device.OnPlatform(20, 0, 0), 0, 0);
    this.BackgroundImage = "background.png";

    // Build the page.
    this.Content = mainLayout;
}

}
I don't understand why its trying to cast MyPickerModel to PickerRenderer+PickerSource

Thanks!